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SODIUM SILICATE  (WATER  GLASS)

103

Donates, chlorides, sulphates and arsenic (tests 1-4).    The content in phos-
phate may be deduced from the determination of the phosphoric acid (5).

1.  Solubility.—i part in 10 of water should give a clear, colourless
solution.

2.  Carbonates.—Any evolution of gas noted on dropping a few crystals
into dilute hydrochloric acid.

3.  Chlorides, Sulphates.—The i : 10 solution, acidified with dilute
nitric acid, is tested with silver nitrate or barium chloride.

4.  Arsenic.—i gram, with 5 c.c. of Bettendorf's reagent, should give
no coloration within an hour.

5.  Determination of the Phosphoric Acid.—25 grams are dissolved
in water to i litre and in 20 c.c. of this solution (= 0-5 gram of substance)
the phosphoric acid precipitated with magnesia mixture (see Fertilisers :
Determination of Phosphoric Acid, A) and the magnesium pyrophosphate
weighed as usual :  i gram Mg2P207 = 3-216 grams Na2HP04 + I2H20 =
0-6376 gram P.205-

Commercial sodium phosphate of good quality contains about 98% Na2HPO4
+ i2H2O or 19-4-19-5% P2O5.

SODIUM   SILICATE (Water Glass)

This has no well defined and constant composition, but usually consists
of a mixture of sodium tri- and tetra-silicates, Na2Si307 and Na2Si409.
It is sold as a colourless, yellowish or greenish solution (30-33°, 37-40° or
50° Baume), or in powder or glass-like masses. It dissolves in water (the
solid, containing a large excess of silica, only with difficulty), the aqueous
solution giving a gelatinous precipitate of silica with acids.

In either fhe liquid or solid form, chlorides, sulphates, alumina and
insoluble substances should be tested for, and the silica and alkali deter-
mined :

1.  Solubility.—-20 grams diluted to 500 c.c. should give a clear liquid
which does not become turbid after some days.

2.  Chlorides, Sulphates.—1-2 grams, diluted with 100 c.c. of water,
acidified with nitric acid and filtered, is tested with silver nitrate or barium
chloride.

3.  Alumina, Ferric Oxide, etc.—2 grams are evaporated with excess
of cone, hydrochloric acid and the residue dried at 120° to render the silica
insoluble.    The residue is taken up in dilute hydrochloric acid, the solution
filtered and the filtrate tested for aluminium, iron, and any lime or other
extraneous substance.

4.   Quantitative Determinations.—100 grams are diluted with water
to I litre and the solution used for the following determinations :

(a) TOTAL ALKALI. 50 c.c. (== 5 grams of substance) are titrated with
N-hydrochloric acid in presence of methyl orange : i c.c. N-acid = 0-031
gram Na20.

(6) SILICA. 50 c.c. (== 5 grams of substance) are decomposed with cone.
HC1 in a platinum dish, the liquid being evaporated to dryness, the residuedepends on